不能更新数据 [ 2.0 版本 ]

/**
 * 重置密码
 * @return [type] [description]
 */
public function resetPassword()
{
    $res = self::findOne(['user_mobile' => $this->user_mobile]);
    if ($res) {
        $res->user_passwd = md5($this->user_passwd);
        if ($res->save()) {
            return true;
        } else {
            return false;
        }
    } else {
        return false;
    }
    
}

$res是有数据的

frontend\modules\appbuild\models\AppUser Object
(
    [user_passwd_check] => 
    [verifyCode] => 
    [_attributes:yii\db\BaseActiveRecord:private] => Array
        (
            [id] => 16
            [agent_id] => 0
            [user_mobile] => 18336070000
            [user_passwd] => 4297f44b13955235245b2497399d7a93
            [user_qq] => 
            [user_email] => 
            [user_money] => 0.00
            [user_score] => 0
            [user_level] => 1
            [created_at] => 1482833823
            [status] => 1
        )

    [_oldAttributes:yii\db\BaseActiveRecord:private] => Array
        (
            [id] => 16
            [agent_id] => 0
            [user_mobile] => 18336070000
            [user_passwd] => e10adc3949ba59abbe56e057f20f883e
            [user_qq] => 
            [user_email] => 
            [user_money] => 0.00
            [user_score] => 0
            [user_level] => 1
            [created_at] => 1482833823
            [status] => 1
        )

    [_related:yii\db\BaseActiveRecord:private] => Array
        (
        )

    [_errors:yii\base\Model:private] => 
    [_validators:yii\base\Model:private] => 
    [_scenario:yii\base\Model:private] => default
    [_events:yii\base\Component:private] => Array
        (
        )

    [_behaviors:yii\base\Component:private] => Array
        (
        )

)

但 $res->save() 是一直是 false